While writing the same parser in JavaScript and Haskell I found these two snippets illustrated to me why monads and do syntax are awesome.

parse.hs

piece :: Parser Piece
piece = do
    many space
    t <- pieceType
    c <- pieceColour
    return $ Piece t c

parse.js

piece = _do(many(space), function(_) {
    return _do(pieceType, function(t) {    
        return _do(colour, function(c) {
            return ret(new Piece(t,c));
        });
    });
});